There has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen to form syngas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch process, carbon monoxide is converted into organic molecules containing carbon and hydrogen, including aliphatic hydrocarbons such as paraffins and/or olefins, as well as oxygenates. Paraffins are particularly desirable as the basis of synthetic diesel fuel. Consequently, in the production of a Fischer-Tropsch product stream for processing to a fuel, it is desirable to maximize the production of high value liquid hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon molecule (C5+ hydrocarbons).
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. A feed containing carbon monoxide and hydrogen is typically contacted with a catalyst in a reactor to form a range of hydrocarbons including gases, liquids and waxes. The composition of a catalyst influences the relative amounts of hydrocarbons obtained from a Fischer-Tropsch catalytic process. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification). For instance, a catalytic metal is typically selected from the group consisting of cobalt, ruthenium, nickel, iron, and combinations thereof. Cobalt metal is particularly desirable in catalysts used in converting natural gas to heavy hydrocarbons suitable for the production of diesel fuel. Alternatively, iron, nickel, and ruthenium have been used in Fischer-Tropsch catalysts.
Further, in addition to the catalytic metal, a Fischer-Tropsch catalyst often includes a support material. The support is typically a porous material that provides mechanical strength and a high surface area, in which the active metal and promoter(s) can be deposited. In a common method of loading a Fischer-Tropsch metal to a support, the support is impregnated with a solution containing a dissolved metal-containing compound. The metal may be impregnated in a single impregnation, drying and calcination step or in multiple impregnation steps. When a promoter is used in the catalyst formulation, an impregnation solution may further contain a promoter-containing compound. After drying the support, the resulting catalyst precursor is calcined, typically by heating in an oxidizing atmosphere, to decompose the metal-containing compound to a metal oxide. When the catalytic metal is cobalt, the catalyst precursor is then typically reduced in hydrogen to convert the oxide compound to reduced “metallic” metal. When the catalyst includes a promoter, the reduction conditions may cause reduction of the promoter, or the promoter may remain as an oxide compound.
Catalyst supports for catalysts used in Fischer-Tropsch synthesis of hydrocarbons have typically been refractory oxides, e.g., silica, alumina, titania, zirconia or mixtures thereof (see E. Iglesia et al. 1993, In: “Computer-Aided Design of Catalysts,” ed. E. R. Becker et al., p. 215, New York, Marcel Dekker, Inc.). In particular, various aluminum oxide compounds have been used as catalyst supports and continue to be improved. For example, gamma-alumina is an oxide compound of aluminum having, in its pure form, the empirical formula γ-Al2O3. Gamma-alumina distinguished from other polymorphic forms of alumina, such as alpha-alumina (α-Al2O3), by its structure, which may be detected for example by x-ray diffraction (see for example Zhou & Snyder, 1991, Acta Cryst., vol B47, pp. 617-630) or electron microscopy (see for example Santos et al., 2000, Materials Research, vol 3, No. 4, pp. 101-114). The structure of gamma-alumina is conventionally thought to approximate a spinel with a cubic form or a tetragonal form or combination.
In a common method of producing gamma-alumina, naturally occurring bauxite is transformed to gamma-alumina via intermediates. Bauxite is an ore, which is obtained from the earth's crust. Minerals commonly found in bauxite and the empirical formulas of their pure forms include gibbsite (α-Al2O3.3H2O), boehmite (α-Al2O3.H2O), diaspore (β-Al2O3.H2O), hematite (α-Fe2O3), goethite (α-FeOOH), magnetite (Fe3O4), siderite (FeCO3), ilmenite (FeTiO3), anatase (TiO2), rutile (TiO2), brookite (TiO2), hallyosite (Al2O3 2SiO2.3H2O), kaolinite (Al2O3 2SiO2 2H2O), and quartz (SiO2).
In a first transformation, gibbsite is derived from bauxite. The Bayer process is one common process for producing gibbsite from bauxite. The Bayer process was originally developed by Karl Joseph Bayer in 1888 and is the basis of most commercial processes for the production of gibbsite. As it is conventionally carried out, the Bayer process includes digestion of bauxite with sodium hydroxide in solution at elevated temperature and pressure to form sodium aluminate in solution, separation of insoluble impurities from the solution, and precipitation of gibbsite from the solution.
In a second transformation, boehmite is derived from gibbsite. As disclosed above, gibbsite is a trihydrated alumina having, in its pure form, the empirical formula α-Al2O3.3H2O. Transformation of gibbsite to boehmite may be accomplished by varying the conditions so as to influence the thermodynamic equilibrium to favor boehmite. For example, a method for producing boehmite from gibbsite may include dehydration in air at 180° C.
In a third transformation, gamma-alumina is derived from boehmite. Boehmite, in its pure form has the empirical formula α-Al2O3.H2O. Alternately, it is denoted in the art by γ-AlO(OH). The respective α and γ prefixes refer to the crystalline form. Boehmite is distinguished from other polymorphic forms of monohydrated alumina, such as diaspore (β-Al2O3.H2O), by its structure or crystalline form. In particular, boehmite typically has orthorhombic symmetry. Transformation of boehmite to gamma-alumina may be accomplished by varying the conditions so as to influence the thermodynamic equilibrium to favor gamma-alumina.
A support material is desirably stable. Under ambient (standard) conditions of temperature and pressure, such as for storage, gamma-alumina is less reactive and therefore more stable than boehmite. Thus, gamma-alumina is typically regarded as a more desirable support material than boehmite. Further, calcination of boehmite to form gamma-alumina before loading the catalytic metal to the gamma-alumina is generally regarded as a desirable step in the formation of a catalyst on alumina. Therefore, the catalytic metal is typically not loaded to boehmite itself in forming the catalyst.
Despite the tendency of gamma-alumina to be stable at atmospheric conditions, gamma-alumina is known to exhibit a tendency to instability under hydrothermal conditions. For example, M. Abso-Haalabi, et al. in “Preparation of Catalysts V”, Ed. G. Poncelet, et al. (1991, Elsevier, Amsterdam, pp. 155-163) disclose that gamma-alumina undergoes an increase in average pore size and an accompanying decrease in surface area after hydrothermal treatment in the temperature range 150-300° C. Such a transformation would be undesirable in a catalyst. However, similar hydrothermal conditions occur, for example, in the Fischer-Tropsch process. In particular, in a Fischer-Tropsch process, water is produced during the Fischer-Tropsch reaction. The presence of water together with the elevated temperatures conventionally employed in the Fischer-Tropsch process create conditions in which hydrothermal stability, which is stability at elevated temperatures in the presence of water, is desirable. Fischer-Tropsch catalysts using gamma-alumina supports are known to exhibit a tendency to hydrothermal instability under Fischer-Tropsch operating conditions. This instability tends to cause a decrease in performance of gamma-alumina supported catalysts.
Finely divided supported catalysts used in fluidized or slurry systems have been known to attrit and deactivate, which causes longevity concerns and product separation issues due to fines formation. The attrition and the deactivation may be due in part to hydrothermal degradation by high pressure and temperature steam from water formed in the reactor. Particularly, the high pressure and temperature steam may promote rehydration of the catalyst support, such as in the case of an alumina support to boehmite and/or gibbsite phases causing a change in the chemical structure and leading to structural instability.
Consequently, there remains a significant need to enhance the robustness of a Fischer-Tropsch supported catalyst without jeopardizing its performance by optimizing the alumina-based support structure by careful selection of the alumina precursor and by addition of a structural stabilizer so as to provide a good performing Fischer-Tropsch catalyst with improved overall physical properties and increased hydrothermal stability under Fischer-Tropsch operating conditions. Additional needs include increased porosity of the support structure of a Fischer-Tropsch supported catalyst to improve diffusional limitations while keeping its surface area sufficient to provide optimal dispersion of the Fischer-Tropsch metal(s).